Why speed isn't everything

Semiconductors: When it comes to designing chips, making them go faster is no longer the most significant challenge. Is Moore's Law dead?

IT SEEMS almost obligatory to begin with Moore's Law. The observation made in 1965 by Gordon Moore—that the number of transistors on a chip would grow exponentially—has proven remarkably resilient. (Dr Moore went on to co-found Intel, which is now the world's largest chipmaker.) Each time it seems that the end is in sight, a new technique is devised to make transistors still smaller and cram more of them on to a chip.

The first transistor, built in 1947, was a few inches square and half an inch high. In 1959, two groups separately realised that instead of painstakingly making transistors one at a time, many of them could be created at once by etching conducting pathways on a wafer of silicon, which had the necessary electrical properties.

This paved the way for the modern computer. Once the concept was proven, it was just a question of progressive miniaturisation—the pace of which Moore's Law so optimistically, yet correctly, predicted. Today, transistors are etched into an ultra-thin layer on a microchip, a square inch of which can contain many tens of millions of them.

The first computer chips were designed to execute a series of straightforward operations, one after another, such as adding together or comparing two quantities. The chip could also store results and retrieve them again from a separate memory, which also held the program it was running. Then, as now, the movement of data to and fro, both inside the chip and between the chip and the memory, was co-ordinated using an oscillating crystal, which sends out a periodic signal like a ticking clock.

Over the years, the frequency of this signal, known as the clock speed, became the key measure of processor performance. In parallel with Moore's Law, which predicts that the number of transistors on a chip increases exponentially, the clock speed has done the same, doubling roughly every 18 months from thousands of ticks per second in 1971, to millions in the 1980s, to billions today. But while optimists believe that this process will continue, chip developers across the industry now agree that clock speed will no longer be the key metric of processor performance, for several reasons.

Parallel lives

The first is the growth of parallelism—the practice of getting a chip to execute many different operations simultaneously. In the past, this was confined to the realm of high-end supercomputers, as a way of improving their performance. But it is now becoming common in personal computers, and is bound to become more so. As a result, the amount of processing a chip can perform with each tick of the clock will be just as important as the frequency at which the clock is ticking.

A driving factor behind this parallelism is the fact that, while processor speed has increased with such remarkable rapidity, the speed of memories has lagged. Marc Tremblay of Sun Microsystems, a computer-maker based in Santa Clara, California, says the gap between processor speed and memory speed is likely to grow. Parallelism within a single chip allows several different processing units to share the same memory, so the memory's slowness is not such a problem.

This is because the limiting factor is not so much the throughput of memory chips (the rate at which data can be moved in and out of them) but the administrative overhead associated with moving information in and out of the processor. Because of this, chip designers can gain by putting several distinct processors on the same chip, and have them share a fast, local memory inside the chip itself. This approach is known as multiple cores, or multi-core for short. A related approach is known as simultaneous multithreading. It involves modifying a single processor to enable it to switch quickly between several distinct tasks. While one task is waiting for data to arrive from the main memory, another can continue to execute—so a single processor can, in effect, do the work of many.

A second reason why clock speed will no longer be an accurate measure of performance is that distributing the clock's signal to all the different parts of a chip is more difficult than it sounds. Jo Ebergen, an engineer at Sun, says that reducing the “skew” on a chip—the amount by which clock signals might be out of synch—takes a very skilful chip designer. It is, he says, as much an art as a science. And it is becoming more difficult as chips get larger and more complex.

Tick, tock

That is why Sun is aggressively exploring “asynchronous” technology, which involves getting rid of the clock entirely. This approach has costs as well as benefits, since miniature circuits known as “rendezvous circuits” must be placed at circuit junctions to co-ordinate the flow of data. It is rather like replacing a city-wide network of traffic lights with policemen at every corner. But, says Dr Ebergen, in one recent experiment with a test chip that could run in both synchronous and asynchronous modes, the asynchronous mode won out. That is because in a synchronous design, every operation must wait for the slowest one to complete, while in an asynchronous one, a laggard only delays the local part of a calculation.

Clockless chips, says Dr Ebergen, also have the added benefit of emitting far less radio interference. So asynchronous circuits could be particularly useful in devices such as mobile phones, where radio interference is a substantial concern. Wilf Pinfold, the director of microprocessor research at Intel, points out that opinions over the value of asynchronous design are quite divided. But it seems clear that, at least in a portion of the market, it will become more and more important.

Finally, getting chips to run at higher clock speeds is diminishing in importance because another problem is becoming more pressing: getting them to consume less power. Fred Weber, the chief technical officer of AMD, a chipmaker based in Sunnyvale, California, says power consumption is now the biggest problem in chip design, for several reasons. The first is the growing prevalence of mobile devices, such as laptop and handheld computers. Increasing the battery life of such devices is in some cases more important than squeezing an extra bit of speed out the system.

A related problem is heat. Today's fastest PC microprocessors consume about 100 watts of electrical power, the same as a bright light bulb. But light bulbs get hot—too hot to have inside a desktop computer, and far too hot for the inside of a laptop. That is why desktop computers have noisy fans, and laptop computers are never as fast. Ghavam Shahidi, who works at IBM Research in East Fishkill, New York, predicts that high-end PCs may well come to rely upon novel techniques, such as water-cooling systems. But for PCs aimed at the mass market, fancy cooling systems will make desktop machines too expensive and laptops too bulky.

So designers are now striving to minimise the power consumption of their chips, with speed as an ancillary consideration. Dr Pinfold says one solution his team is exploring is to use multiple cores, switching from one to another not to increase speed, but rather to minimise the total heat generated. When one core gets too hot, it is switched off for a while to cool down, while another core takes over.

Into the third dimension

All of these ideas are already, to a greater or lesser degree, finding their way into existing microchips. For example, Philips, a Dutch electronics manufacturer, has already built a pager that uses asynchronous technology. Intel already sells chips capable of simultaneous multi-threading, and Sun's UltraSparc IV chip, launched in February, incorporates both multiple cores and multi-threading.

What next? One problem, says Dr Ebergen, is that as the performance of individual chips in a computer improves, the limiting factor on the system's overall performance becomes the “interconnect” between the chips. This currently consists of pins—little wires, essentially—protruding from the edge of the chip. Although the number of these pins has increased, it has not increased nearly as fast as the number of transistors. The result is that a chip with billions of transistors must make do with just a few hundred pins to communicate with its neighbours.

One solution that Sun is investigating is called “proximity communication”. Rather than using wires to connect two chips, the chips would instead be placed very close together. A build-up of electric charge on one chip then induces a corresponding build-up on the other, by a process known as capacitive coupling. If Sun can work out all the kinks, this approach would increase the density of the interconnect, improving the overall system performance. Dr Tremblay says the technology may be available as soon as 2007, at least at the high end of the market.

Dr Weber, at AMD, sees another way forward. In the past, he says, the limiting factor on chip performance was the transistors themselves—getting them to be small and fast enough. But transistor design has progressed so much that, although further progress is certainly possible, the thing holding chips back today is not the transistors but the wires—the metal paths etched on to the chip—that electrons must travel along to get from one part of a chip to another.

One way of speeding things up would be to make chips in three dimensions, rather than on a flat plane as they are today. Dean McCarron, an industry analyst at Mercury Research, points out that this is already happening to some extent, since wires can cross over one another on a chip, though no actual transistors are stacked on top of each other. Even so, a true three-dimensional chip is still some way off, as the necessary etching technology is still in its infancy. Dr Weber acknowledges that there will also be design challenges to overcome. For example, heat dissipation from the interior of such a chip will be even more difficult to manage than with existing designs.

So does all this spell the end of Moore's Law? Its demise has, after all, been predicted many times in the past. There is no simple answer. On the one hand, it seems the law, at least as it relates to increases in transistor density, will continue to hold for some time. On the other hand, the law's significance is likely to diminish, as computer-buyers demand more than just speed from their machines—and chip designers tailor their wares accordingly.